How does a spacecraft get to where its going?
Satellites put in space by people serve different purposes. Some use orbits to move from planet to planet. Others stay moving around one planet to do a specific job. The kinds of orbits they travel in help them to achieve this purpose. Some kinds of planetary orbits include:
Geosynchronous Orbits. A geosychronous orbit (GEO) is a circular, low orbit about Earth having a period of 23 hours 56 minutes 4 seconds--that is, the same amount of time it takes for the Earth to turn, so as the Earth spins, the satellite moves in time with it. Geosynchronous means "in time with the Earth." A spacecraft in geosynchronous stays over the same line of longitude. (A line of longitude marks one slice of the world from north to south pole.) Often a satellite in geosynchronous orbit stays above the same spot on Earth. When it does, it is called geostationary. This orbit is ideal for certain kinds of communication satellites, or meteorological (weather) satellites that have a job to do over one part of the world.
Polar Orbits. Polar orbits are useful for spacecraft that carry out mapping or surveillance operations. A satellite in polar orbit goes around the Earth from pole to pole. The planet spins underneath it as the satellite goes from north to south. This gives the spacecraft access to virtually every point on the surface. The Magellan spacecraft used a nearly-polar orbit at Venus. When the planet rotated once, all 360 degrees longitude had been exposed to Magellan's surveillance.
Walking Orbits. There are some things that interfere with making spacecraft follow perfect orbits easily. Planets are not perfectly spherical (ball shaped) and they do not have evenly distributed mass. Some parts of the planet might weigh a little more than others. For example a huge iron concentration could be in one part of the planet, making that side weigh a little more. Also, gravity can be uneven in space. Other bodies such as the Sun, the moon or other satellites, pull on spacecraft in orbit about a planet. Sometimes, scientists choose the path of a spacecraft's orbit to use this other gravity to slowly change the orbit over time. The result is called a walking orbit.
Suns-Synchronous Orbits. Sometimes a walking orbit can be designed so that the orbit changes slowly in time with the planet moving around the Sun, and in time with the planet's rotation so that the spacecraft is always at the same angle to the Sun. This is called a Sun-synchronous orbit. On Earth, this would work out so that the orbit always passes a low point at the same local time every day. This can be useful if instruments on board depend on a certain angle of solar illumination on the surface. Mars Global Surveyor's intended orbit at Mars is a 2-PM Mars local time Sun-synchronous orbit.
How do we put a spacecraft into orbit?
Once a ship is in orbit, do we have to do anything to keep it there?
What is an orbit?
What causes an orbit to happen?
What is a satellite?
Why is it a good idea to launch a ship into orbit from near the equator?
What role does the Sun play in space missions like DS1's?
What is mass?
How do spacecraft use an orbit to move from planet to planet?
What could cause an orbit to fail?